Method of producing heatable mirrors by depositing coatings on glass

ABSTRACT

A method of producing heatable mirrors comprising depositing onto a ribbon of hot glass during the production process a reflecting coating whereby the mirrors so formed have a visible light reflection of at least 70% and depositing an electroconductive heating layer onto the mirrors. There is also provided a heatable mirror comprising a glass substrate carrying a non-metallic reflecting coating whereby the mirror has a visible light reflection of at least 70% and an electroconductive heating layer deposited on the coated substrate.

This application is a division of application Ser. No. 08/370,410 filedJan. 9, 1995 which application is now U.S. Pat. No. 5,576,885.

BACKGROUND OF THE INVENTION

The invention relates to heated mirrors and their manufacture.

The light reflecting properties of mirrors are generally provided by alayer of highly reflecting metal, especially silver, aluminium orchromium, applied to a glass or plastics substrate; copper layers aresometimes used as an alternative, but are generally less acceptablebecause of the strong red tint of the reflected light.

Silver coatings are generally applied to preformed glass plates, in thecold, by wet chemical methods in which a solution of silver salt isapplied to the glass surface and reacted with a reducing agent whichreduces silver ions present to silver metal which deposits on the glasssurface. The silver used is not very durable in use and in practicerequires protection by other layers, and these methods are generallyunsuitable for application to glass on the production line on which itis formed so that a separate "silvering" line is required to produce thesilvered glass.

Aluminium coatings are difficult to apply by chemical methods because ofthe strongly reducing nature of aluminium metal, and aluminium mirrorsare generally produced by deposition methods carried out at low pressuree.g. by sputtering. Such low pressure methods are essentially batchprocesses and, like the wet chemical methods used for deposition ofsilver mirrors, are generally unsuitable for on-line application on theproduction line on which the glass is made.

GB 2248853A discloses a method of coating glass with aluminium to form amirror. A solution of an alane amine adduct of aluminium is formed andthe liquid is deposited onto heated glass. The adduct decomposes to forman aluminium coating. Although it is stated that it is envisaged thatthe invention may be used in conjunction with float glass production,there is no exemplification of such a use. It is believed thatsubstantial technical problems could be encountered in simplyintroducing the disclosed aluminium compounds into a float glass line.

Silicon layers have also been used to produce reflecting layers (which,like silver and aluminium layers, are substantially neutral inreflection colour) on architectural glazing for aesthetic and solarcontrol purposes. GB 1507465, 1507996 and 1573154 relate to a continuouschemical vapour deposition method for producing float glass having sucha silicon layer, and U.S. Pat. No. 4661381 describes a development ofthat method. However, such silicon layers do not provide the highreflections commonly required in mirrors. Thus REFLECTAFLOAT (trademark) glass, commercially available from Pilkington Glass Limited of St.Helens, England, has a reflection of about 50%, and MIRROPANE EP (trademark) commercially available from Libbey-Owens-Ford Co. has a reflectionof about 60%.

None of the above technology is currently suitable for the applicationof highly reflecting coatings to glass during the glass productionprocess to provide a coated glass substrate with a light reflection ofover 70%, and preferably over 80%.

Mirrors are often used in situations, such as in domestic bathrooms oras side view automotive mirrors, where water vapour can condense out onthe mirror surface thereby to steam or mist up the mirror or water orice can be deposited on the mirror. It is known to provide silveredmirrors having disposed behind the mirror a heating assembly comprisinga heating element, assembled in or on an insulating layer. An example ofsuch a known arrangement is a heating wire assembled in or on a plasticsfilm which is adhered onto the rearmost paint layers of the mirror, theheating element being connected to a source of electrical power. Such aheating assembly is relatively complicated and can be expensive tomanufacture.

The present invention aims to provide an improved heated mirror andmanufacturing method therefor.

On a completely different scale, it has been proposed in GB 1262163, toproduce very highly reflecting (greater than 90%) "cold light" mirrorscomprising silicon layers for use, for example in cinema projectors, forseparating heat radiation from visible light. Such cold light mirrorsare produced by vacuum deposition on thin bases, typically glasssubstrates 3 mm thick or less, and are used without any backing paint tominimise build up of heat in the glass. GB 1262163 refers, in discussingthe prior art, to a known cold light mirror comprising a "purest siliconlayer" covered by four to six alternate layers of silicon oxide andtantalum oxide or titanium oxide but concludes that, for a satisfactoryproduct, substantially more layers would be required. It thereforeproposes to achieve the very high reflection (greater than 90%) requiredin a different way using several silicon layers as the individual layersof high refractive index of a multi-layer interference system.

Much more recently, it has been proposed by J. Stone and L. W. Stulz(Applied Optics, February 1990, Volume 29, No. 4) to use quarterwavelength stacks of silicon and silica layers for mirrors in thespectral region between 1.0 and 1.6 microns (i.e. within the infra red).However, the authors observe that silicon cannot be used at wavelengthsbelow about 1 micron (and thus not in the visible region of thespectrum) due to its high absorption at such wavelengths. Stone andStulz refer to the deposition of Si/SiO₂ by low pressure methods such asreactive sputtering and electron beam evaporation.

Although GB 1262163 and the Stone and Stulz paper are discussed herein,the technology, in particular the production process described therein,is not suitable for the production of on line glass mirrors whichessentially requires processes suitable for use at atmospheric pressure.Accordingly, these references would not be considered by the personskilled in the art as being in any way relevant to the production ofon-line mirrors to compete with the conventional "off-line" mirrorsdiscussed above.

SUMMARY OF THE INVENTION

The present inventors have discovered in accordance with one aspect ofthe present invention that highly reflecting coatings can in practice beapplied on line to glass during the production process, especially on afloat glass production line, by depositing a reflecting layer and bydepositing, before or after the deposition of the reflecting layer, twolayers as reflection enhancing layers. A heating layer comprisingelectrically conductive oxide film can also be deposited on-line and soa heated mirror can be made during the production process.Alternatively, the heating layer can be applied off-line.

According to the present invention there is provided a method ofproducing heatable mirrors comprising depositing onto a ribbon of hotglass during the production process a non-metallic reflecting coatingwhereby the mirrors so formed have a visible light reflection of atleast 70% and depositing an electroconductive heating layer onto themirrors.

The coated ribbon is cut on-line and will usually be further cutoff-line to provide separate mirrors of the required size.

The present invention further provides a heatable mirror comprising aglass substrate carrying a non-metallic reflecting coating whereby themirror has a visible light reflection of at least 70% and anelectroconductive heating layer deposited on the coated substrate.

In one preferred embodiment the heating layer is deposited over thereflecting coating onto the ribbon of hot glass during the productionprocess. The reflecting coating may be deposited in a float bath of afloat glass plant and the heating layer may be deposited in a gapbetween the float bath and an annealing lehr. The heating layer sodeposited may comprise the rear surface of a back surface mirror.

In an alternative embodiment, the heating layer is deposited on a glasssurface which is on the opposing side of the mirror to the reflectingcoating. Such a heating layer may be deposited off-line onto mirrorswhich have been cut from the coated ribbon. Such a heating layer sodeposited may comprise the rear face of a front surface mirror.

The reflecting coating may comprise a reflecting layer and at least tworeflection enhancing layers. Preferably, the reflection enhancing layerscomprise an intermediate layer of the coating of relatively lowreflective index and a layer adjacent to the intermediate layer ofrelatively high refractive index.

In this specification the terms "reflecting layer" and "reflectionenhancing layer" are intended to indicate the relative interrelationshipbetween the layer positions. Thus, the reflecting layer is, in use,furthest from the source of light to be reflected and the reflectionenhancing layers are between the light source and the reflecting layer.The term "reflecting layer" is not necessarily intended to imply thatthat layer is the primary contributor to the overall reflection of thereflecting coating as compared to the other layers of the coating. Incertain embodiments the largest contributor to the overall reflectionmay be a reflection enhancing layer. Thus for front surface mirrors theinner of the said three layers is the reflecting layer and intermediateand outer layers act as reflection enhancing layers, and for backsurface mirrors the outer of the said three layers is the reflectinglayer and the intermediate and inner layers act as reflection enhancinglayers. The inner layer is identified as the layer of the coatingnearest to the glass and the outer layer as the layer furthest from theglass of the said three layers.

It is known in the art that refractive index varies with wavelength. Inthis specification and claims, references to "refractive index" areintended to mean (in conventional manner) the refractive index for lightof wavelength 550 nm and, in assessing and quoting refractive indexvalues, any imaginary part of the refractive index is disregarded.

The expression "visible light reflection", as used in the presentspecification and claims, refers to the, percentage of light reflectedunder Illuminant D65 source 1931 Observer Conditions.

The reflecting layer may have a high refractive index and the reflectionenhancing layers may have high and low refractive indices so that theresultant stack of layers has successive high, low and high refractiveindices.

The desired high reflection may be achieved using layer thicknesses suchthat reflections from the interfaces between the said coating layersreinforce reflections from the outer surface of the said outer layer(for front surface mirrors) or the inner surface of the said inner layer(for back surface mirrors). The materials of the inner and outer layersare preferably selected so that the aggregate refractive index of thematerials of the two layers is at least 5.5 when the reflecting layer isof high refractive index.

Silicon is preferably used for at least one of the inner and outerlayers because (a) it may have a particularly high refractive index and(b) it is readily deposited on-line on hot glass, for example, by theprocesses described in GB 1507465, GB 1507996 and GB 1573154.

The refractive index of silicon may be as great as about 5, (see P. J.Martin, R. P. Netherfield, W. G. Sainty and D. R. McKenzie in Thin SolidFilms 100 (1983) at pages 141-147) although lower values are oftenencountered.

It is believed that, in practice, the value varies depending on theprecise physical form of the silicon and the presence of any impurities,for example oxygen, nitrogen or carbon. For the purpose of the presentinvention, the presence of such impurities may be tolerated (and indeed,it is difficult in practice to produce on-line silicon coatings withoutsignificant oxygen and/or carbon incorporation) provided the refractiveindex is not reduced below about 2.8. Thus the term "silicon" as usedherein with reference to layers of relatively high refractive indexrefers to material which is predominantly silicon, but may contain minorproportions of impurities, provided its refractive index is at least2.8.

While its high refractive index and ease of deposition favour the use ofsilicon, the high absorption of silicon leads to a reduction in thereflection. When only one of the inner and outer layers is of silicon,the other (preferably the inner layer for back surface mirrors and theouter layer for front surface mirrors) must be of a material having ahigher refractive index than the intermediate layer (and of at least1.6) and is preferably of low absorption in the visible region of thespectrum. Preferred materials, other than silicon, for a layer ofrelatively high refractive index are materials having a refractive indexin the range 1.9 to 3.0, usually 2.0 to 2.7 and include tantalum oxide,titanium oxide, tin oxide and silicon oxides (including silicon oxidescontaining additional elements, for example nitrogen and carbon). Theamount of such additional elements in silicon oxide can be varied so asto vary the refractive index because the refractive index iscomposition-dependent. The deposited silicon oxides are generally notstoichiometric. In general, the higher the refractive index of amaterial, and the lower its visible light absorption, the more effectiveit will be as a reflecting layer or reflection enhancing layer of highrefractive index; expressed in another way, a reduction in therefractive index of the material may be compensated for by a reductionin its visible light absorption.

The intermediate layer i.e. the reflection enhancing layer adjacent thereflecting layer, which is of relatively low refractive index, has arefractive index lower (and in any event below 3) than that of the innerand outer layers of relatively high refractive index. In general, thelower the refractive index (for a layer of given light absorption) ofthe intermediate layer, the higher the reflection that can be achieved.The layer of relatively low refractive index will usually have arefractive index below about 2, and it is generally preferred to use alayer of refractive index less than 1.8.

It is also preferred to use as the intermediate layer a material whichis substantially non-absorbing in the visible region of the spectrum inorder to increase the total light reflection. A suitable and convenientlayer material is silicon oxide, which may however contain additionalelements such as carbon or nitrogen, and the term "silicon oxide" isused herein to encompass silicon oxides additionally containing otherelements, for example, silicon oxides containing carbon and/or nitrogenand, when used with reference to the intermediate layer, having arefractive index of less than 2. Surprisingly, it is found in practice,that adjacent layers of silicon and silicon oxide can be appliedpyrolytically to the glass without interdiffusion or interactions whichwould cause unacceptable reduction in the refractive index of thesilicon or increase in the refractive index of the silicon oxide; theadjacent layers of silicon and silicon oxide appear to remain, at leastin terms of their optical performance, separate and distinct. However,it may be that at the interfaces of the layers there exist physicallynarrow interaction zones with steep refractive index gradients that donot alter the optical characteristics of the mirror. Another materialwhich may be used for the intermediate layer is aluminium oxide.

Some of the coating materials, especially silicon, which may be used toform the outer layer of high refractive index have limited scratchresistance and, if a more durable product is required, an additionalprotective layer of a harder material, for example of tin oxide, may bedeposited over said outer layer. It will be appreciated that, if such aprotective layer is used on front surface mirrors, it should be of amaterial (and tin oxide and titanium oxide are examples) that has a lowlight absorption in the visible region of the spectrum in order tomaintain the light reflection of the product, and should be of anoptical thickness subtantially different from a quarter wavelength toavoid suppressing the reflection from the outer layer; if used, such aprotective layer will typically have a thickness in the region of 10 nmto 30 nm. An outermost layer, of silicon, titania or the above-describedprotective layer, provides chemical durability to the mirrors. This is areal technical advantage over the known silver mirrors.

The thicknesses of the layers may be selected, in generally known manner(see for example the prior art referred to above), so that thereflections from the interfaces between the intermediate layer ofrelatively low refractive index and the inner and outer layers reinforcereflections from either the outer surface of the said outer layer (forfront surface mirrors) or the inner surface of said inner layer (forback surface mirrors). This will occur for front surface mirrors whenthe said intermediate and outer layers have an optical thickness ofabout n λ/4 and, for back surface mirrors, when said inner andintermediate layers each have an optical thickness of about n Aλ/4wherein, in each case, λ is a wavelength of light in the visible regionof the spectrum, i.e. from about 400 nm to 750 nm and n is an oddinteger; n may be the same or different for each of the said layers, butis preferably 1 in each case.

It is preferable that, when either (or both) the inner layer or theouter layer is of relatively high refractive index material which isnon-absorbing or only weakly absorbing in the visible region of thespectrum, both said inner and said outer layers have a thickness ofabout n λ/4, where n and λ are as defined above. In this way,reflections from, in the case of front surface mirrors, the interfacebetween the inner layer of relatively high refractive index and theglass and, in the case of back surface mirrors, the face remote from theglass of the outer layer of relatively higher refractive index willreinforce the reflections from the interfaces between the coating layersincreasing the overall visible light reflection of the mirrors. On theother hand when both said inner layer and said outer layer are ofmaterial which is highly absorbing in the visible region of thespectrum, the thickness of the layer remote from the light source (thereflecting layer) is less critical, since the amount of light passingback towards the light source after reflection at the side of that layerremote from the source will be much reduced by absorption.

To achieve the desired visible light reflection of 70% the thicknessesof the layers of optical thickness about n Aλ/4 may be selected so thatthe phase differences of the light of a wavelength of aboat 500 nmreflected towards the light source from the interfaces between the saidcoating layers and either (for front surface mirrors) the outer surfaceof the outer layer or (for back surface mirrors) the inner surface ofthe inner layer are all within ±40% of a wavelength and preferablywithin ±20% of a wavelength. The general condition is that all theprimary reflected rays from the interfaces and either, for front surfacemirrors said outer face or, for back surface mirrors said inner face, besubstantially in phase with a phase error not exceeding those percentagevalues. Preferably, each of the reflection enhancing layers (being inthe case of front surface mirrors each of the outer and intermediatelayers and in the case of back surface mirrors the inner andintermediate layers) will have an optical thickness of 125 nm±25%; and,unless the reflecting layer is a metal, or neither inner nor outer layeris non-absorbing or only weakly absorbing in the visible, the reflectinglayer will also have an optical thickness of 125 nm±25%.

The closer the optical thicknesses of the layers are to n.500 nm/4 themore neutral the reflection colour will be, while the closer the opticalthicknesses of the layers are to n.550 nm/4 the higher will be the totallight reflection. However, it will readily be appreciated, by thoseskilled in the art, that the reflection colour can be tuned by varyingthe optical thicknesses of the layers within the range from about onequarter of 400 nm (blue-green reflection) to one quarter of 750 nm(red-yellow reflection); it will also be appreciated that tuning awayfrom about 550 nm will reduce the total visible light reflection of theproduct.

According to the preferred method of the invention, the layers of therequired index are applied to a ribbon of hot glass during the glassproduction process. The depositions may be carried out in a known mannerby liquid or powder spray processes, or by a chemical vapour depositionprocess, and each of the layers may be leposited by a different type ofprocess. The depositions may be pyrolytic involving decomposition of acompound which is a pre-cursor for the material of the desired layer,possibly by reaction with another compound.

In general, it is convenient to use a chemical vapour deposition processto apply any silicon or silicon oxide (which may contain carbon) layersthat may be required. Thus, for example, any silicon layer may bedeposited (directly or indirectly) on the hot substrate by chemicalvapour deposition from a silane gas, conveniently in a gaseous diluent,for example nitrogen. It is generally most convenient to use monosilane,although other silanes may also be used, such as dichlorosilane. Onesuitable process for deposition of such a silicon layer is described inGB 1507996. If desired, for example to improve the alkali resistance ofthe silicon coating, the reactant gas may contain a proportion of agaseous electron donating compound, especially an ethylenicallyunsaturated hydrocarbon compound, for example, ethylene, as additive.

A layer of silicon oxide containing carbon for use as a reflecting layeror a reflection enhancing layer of high refractive index but lowabsorption in the visible may similarly be deposited by chemical vapourdeposition from a silane gas, conveniently in a gaseous diluent, inadmixture with an ethylenically unsaturated hydrocarbon compound, forexample ethylene, using a somewhat higher proportion of ethylene tosilane than is required to produce a silicon layer. Again, the silaneused is conveniently monosilane.

A silicon oxide layer for use as a reflection enhancing layer of lowrefractive index (i.e. an intermediate layer) may similarly be depositedby chemical vapour deposition from a silane gas, conveniently in agaseous diluent, in admixture with oxygen or a source of oxygen. Amixture of a silane and an ethylenically unsaturated hydrocarbon,together with carbon dioxide or an alternative oxygen compound whichserves as a source of oxygen such as a ketone, for example acetone, maybe used. The relative concentrations of silane and the source of oxygenused will depend on the refractive index required; in general, the lowerthe refractive index required, the larger the proportion ofoxygen-containing compound to silane to be used. Again, the silane usedis preferably a monosilane.

For metal oxide layers, such as tin oxide or titanium oxide, either aliquid or powder spray process or a chemical vapour deposition willgenerally be used. Thus, for example, a layer of tin oxide or titaniumoxide may be deposited by chemical vapour deposition by reaction of thecorresponding gaseous metal chloride and water vapour, or by spraying anon-aqueous solution of the metal chloride onto the hot glass in thepresence of water vapour. Thus tin oxide may be deposited by chemicalvapour deposition of components selected from tin tetrachloride andwater vapour, and an organo tin compound such as diethyl tin dichlorideor tetramethyl tin, and oxygen, the oxygen optionally being present inair. The titanium oxide may be deposited by chemical vapour depositionof a titanium alkoxide, such as titanium isopropoxide, optionally in thepresence of water or air.

When applying a reflecting coating layer to a ribbon of float glass, thechemical vapour deposition techniques can conveniently be carried outinside the float bath i.e. where the glass is supported on a moltenmetal bath under a protective atmosphere (but preferably after the glasshas fisished stretching i.e. at a glass temperature below 750° C.), orafter the ribbon has emerged from the float bath. When using a gascontaining monosilane to deposit silicon, silicon oxide containingcarbon, or other silicon oxide layers, it is preferred to carry out thedeposition of that layer in the float bath where the glass is at atemperature in the range 600° C. to 750° C. in order to achieve asatisfactory rate of deposition.

When applying a coating layer to a ribbon of float glass by a liquid orpowder spray process, it will generally be more convenient to depositthe layer after the ribbon of glass has emerged from the float bath.

The electroconductive heating layer which is applied to the mirrorpreferably comprises a heating layer of a conducting oxide, such asfluorine-doped tin oxide, indium tin oxide or other conducting oxide.When the heating layer is applied over the reflecting coating, which isdeposited onto the ribbon of float glass in the float bath of the floatglass plant, the heating layer is preferably applied in the lehr gapbetween the float bath and the annealing lehr. The heating layer may beapplied using the methods and apparatus disclosed in our GB 2227029 andGB 2225343 the disclosures of which are incorporated herein by referencethereto. Alternatively, the heating layer may be applied off-line eitherin a separate coating furnace or by vacuum deposition after separatemirrors have been cut from the ribbon of glass. The heating layer may bedeposited over the uncoated glass surface or the reflecting coating.

The reactants to form a layer of electroconductive fluorine-doped tinoxide comprise stannic chloride (SnCl₄) and a mixture of hydrogenfluoride and methanol together with steam. The stannic chloride reactantis introduced in a carrier gas as a turbulent flow over the ribbon ofglass, and then the remaining reactants, comprising the HF/methanolmixture and steam, are introduced into that flow to form a compositeturbulent flow along the direction of glass movement. The reactantsreact together forming fluorine-doped tin oxide on the reflectingcoating or on the glass surface. The exhaust gases are extracted awayfrom the ribbon of hot glass.

The preferred layers--including silicon, silicon oxide, titanium oxideand (undoped) tin oxide and the heating layer of fluorine-doped tinoxide used in the practice of the present invention may result in acoated glass product which may be annealed in a similar manner to thatknown for annealing of glass bearing a pyrolytic fluorine-doped tinoxide coating for use as a low emissivity coating with the potentialproblems of annealing a glass bearing a coating of silver (astraditionally used in mirrors) being avoided. This means that suchheatable mirrors can readily be produced on-line in a float glassprocess.

After the mirrors carrying the heating layer have been produced asdescribed above, busbars are deposited onto the individual mirrors, forexample by a silk screen printing process, with the busbars preferablybeing composed of a silver metal-containing frit. The printed busbarsare then pre-dried in an oven and are then fired in a furnace toconsolidate the printed layer. Electrical connections are then made tothe busbars using insulated wires and a solder, for example of indium.In use, the heatable mirrors made in accordance with the presentinvention can be mounted to a source of electrical power, for examplefrom a battery or from the electric mains, and electric current passesthrough the electroconductive heating film thereby heating it, andthereby the front surface of the mirror by heat conduction through theglass substrate and producing a demisting surface on the front surfaceof the mirror. A typical sheet resistance of the electroconductiveheating layer is around 14 ohms/square although the sheet resistance maybe varied, in particular reduced, as required depending upon theparticular application of the heated mirror. For a mirror having asquare aspect, dimensions of 160×160 mm and with bus bars 137 mm apart,and carried on a 2 mm thick glass pane, a typical voltage which isapplied to the electroconductive heating film is 6 to 12 volts at acurrent of 0.47 to 0.9 amps. This generates sufficient heating of thefront surface of the mirror to produce a demisting surface at normalroom temperatures. The voltage and current may be varied depending uponthe heating requirements and the available electrical power.

The process of the present invention is useful for the production ofheatable mirrors for a wide range of purposes, including domestic use asmirrors in bathrooms and bedrooms. For many uses the mirrors will beprovided with an obscuring layer, preferably a substantially opaquelayer, on the side which is to be remote from the source of light to bereflected in use. Thus, for back surface mirrors, the obscuring layerwill usually be applied over the heating layer which extends over thereflecting coating while for front surface mirrors the obscuring layerwill generally be applied over the heating layer which extends over theback surface of the glass.

The ability to produce heatable glass mirrors on-line at high yield,using coating steps based on known technology, for example the pyrolyticdeposition of a silicon layer, is an important step forward.

The skilled man will also appreciate that additional low and highrefractive index quarter wave (n λ/4 where n is an odd integer,preferably 1) layers may be added to the stack of layers to furtherenhance the reflection.

It may also be possible to incorporate additional non-quarter wavelayers between the said inner and outer layers, although in that eventsuch layers are generally best regarded as forming part of a compositeintermediate layer which should, considered as a composite single layer,have a thickness such that the phase differences of the light reflectedtowards the light source from the interfaces of said compositeintermediate layer and the other coating layers and either (for a frontsurface mirror) the outer surface of the outer layer or (for a backsurface mirror) the inner surface of the inner layer are all within ±40%of a wavelength, and preferably within ±20% of a wavelength. Thus thecomposite single layer will have a refractive index less than therefractive index of either said inner layer or said outer layer and lessthan 3; preferably such composite single layer will have a refractiveindex of less than 1.8 and an optical thickness of 125 nm±25%.Similarly, an additional layer may be included between the inner layerand the glass although, in the case of a back surface mirror, it willthen normally be of refractive index intermediate between the refractiveindex of the inner layer and the glass.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is illustrated but not limited by the following drawingsand Example. In the drawings:

FIG. 1 is a section (not to scale) through a mirror in accordance with afirst embodiment of the invention in use as a front surface mirror.

FIG. 2 is a section (not to scale) through a mirror in accordance with asecond embodiment of the invention in use as a back surface mirror.

FIG. 3 is a diagrammatic representation of the arrangement of coatingstations on a float glass production line for production of heatablemirrors in accordance with an embodiment of the method of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a front surface glass mirror comprises a floatglass substrate 1 carrying a coating 2 comprising an inner layer 3 ofrelatively high refractive index, for example of pyrolytic silicon, andintermediate layer 4 of relatively low refractive index, for example ofsilicon oxide having a refractive index below 1.8 and containing siliconand oxygen in atomic proportions of about 1:2, and an outer layer 5 ofrelatively high refractive index, for example of pyrolytic silicon. Ifonly one of the layers 3 and 5 of relatively high refractive index is ofsilicon, it will usually be the inner layer, with a material having alower absorption for visible light, for example silicon oxide containingcarbon or titanium oxide, being used as the outer layer 5. Each of theintermediate layer 4 and the outer layer 5 has an optical thickness ofnλ/4, wherein n is an odd integer (preferably 1) and λ is a wavelengthof light in the visible region of the spectrum i.e. from about 400 nm to750 nm. If the inner and outer layers 3 and 5 are of an absorbingmaterial such as silicon, the thickness of the inner layer is lesscritical, but it may also correspond to an optical thickness of nλ/4wherein n and λ are as defined above and n is an odd integer preferably1.

A protective layer 6 more durable than outer layer 5 is applied overlayer 5. The protective layer may be of tin oxide, and may be applied bychemical vapour deposition. When the outer layer 5 is of silicon, such aprotective layer of tin oxide should be applied only after a surfacelayer of silicon oxide has been formed on the silicon, for example, asdescribed in U.S. Pat. No. 4,661,381.

An electroconductive heating layer 7 is disposed on the back surface ofthe glass substrate 1, the heating layer preferably comprising a coatingof fluorine-doped tin oxide. The thickness of the heating layer 7 istypically around 3200 angstroms. The heating layer 7 typically has asheet resistance of around 14 ohms/square or lower although the sheetresistance may be varied as required depending upon the end applicationof the heatable mirror. Busbars 8 preferably composed ofsilver-containing frit are silk screen printed on opposed sides of theheating layer 7. Electrical connections (not shown) are made to thebusbars 8, for example by using insulated wires and an indium-basedsolder. An obscuring layer 9 which may be an opaque layer of backingpaint, which may be a conventional mirror backing paint, applied overthe heating layer 7 on the back surface of the glass 1.

Referring to FIG. 2, a back surface glass mirror comprises a float glasssubstrate 11 carrying a coating 12 comprising an inner layer 13 ofrelatively high refractive index, for example of pyrolytic silicon,silicon oxide, tin oxide or titanium oxide, an intermediate layer 14 ofrelatively low refractive index, for example of silicon oxide having arefractive index below 1.8 and containing silicon and oxygen in atomicproportions of about 1:2, and an outer layer 15 of relatively highrefractive index. The outer layer 15 preferably comprises a layer ofsilicon. As in FIG. 1 busbars 18 are deposited onto the heating layer16. Each of the inner layer 13 and intermediate layer 14 has an opticalthickness of n λ/4, wherein n is an odd integer (preferaly 1) and λ is awavelength of light in the visible region of the spectrum i.e. fromabout 400 nm to 750 nm. If the inner and outer layers 13 and 15 are ofan absorbing material such as silicon, the thickness of the outer layeris less critical, but it may also correspond to an optical thickness ofn λ/4 wherein n and λ are defined above and n is preferably 1.

The use of titanium oxide as one of the inner or outer layers instead ofsilicon has been found to increase the reflectivity of the mirrorproducts. For example, for back surface mirrors, the use of titaniumdioxide as the inner layer can increase the reflectivity, as compared tosuch mirrors having a silicon inner layer, by about 3 to 7%.

An opaque layer 19 of backing paint, which may be a conventional mirrorbacking paint applied over the heating layer 16 on glass substrate 11 asan obscuring layer.

FIG. 3 illustrates diagrammatically, a float glass production linecomprising a glass melting section 21, a float bath section 22 forforming the molten glass into a continuous ribbon, a lehr section 23 forannealing the said glass ribbon and a warehouse section 24 for cuttingpieces of glass from the ribbon for storage and/or distribution and use.For the production of mirrors in accordance with the method of theinvention, each of the three coating stations for respectively applyingthe inner, intermediate and outer layers will normally be located in orbetween the float bath section 22 and lehr section 23; in theillustrated embodiment of the invention, the said three coating stations25,26,27 are arranged in the float bath section 22 as shown in FIG. 3.In the illustrated embodiment, a heating layer deposition section 28 islocated between the float glass section 22 at which the reflectingcoating is applied to the glass substrate and the lehr section 23. Theheating layer deposition section 28 may have the same structure as thecoating apparatus disclosed in GB 2225343. This section is provided withreactants as described above in order to enable a heating layer, such afluorine-doped tin oxide, to be deposited onto the reflecting coating.The float glass production line of FIG. 3 is specifically arranged toproduce a back surface mirror having a reflecting coating on the backsurface of the glass and a heating layer on the back surface of thereflecting coating. In alternative embodiments, one or each of thecoating stations for applying inner, intermediate and outer layers inaccordance with the invention may be located between the float bathsection 22 and the lehr section 23. The location of each coating stationis selected to be at a position where the glass ribbon has substantiallyreached its final thickness (usually at a glass temperature of around750° C.) so that it is not subject to further stretching which mightcrack any coating applied, but where its temperature remainssufficiently high for formation of a further pyrolytic layer (usually aglass temperature of at least 300° C.).

The heating layer applying station 28 is required to be downstream inthe direction of glass flow from the coating stations for applying theinner, intermediate and outer layers to the reflecting coating. Theheating layer is preferably deposited at a position where the glasstemperature is around 600° C.

The following Example illustrates the present invention without limitingit, and in the Example mirrors were produced on-line using a float glassproduction line having the structure shown in FIG. 3 (but without theheating layer deposition section 28) and a heatable layer wassubsequently applied over the reflecting coating of the mirrors in anoff-line deposition furnace.

EXAMPLE 1

Glass mirrors, intended for use as back surface heatable mirrors, wereproduced using the laminar vapour coating process and apparatusdescribed in GB 1507996 incorporating the modification described in GB2209176A. Three separate coating beams, each as described in said patentspecifications, were used to apply successive silicon, silicon oxide andsilicon layers to a ribbon of float glass. Each of the three coatingbeams was located in the float bath where the glass ribbon was supportedon a bath of molten metal. The upstream beam was fed with 0.4 liters perminute of monosilane and 36 liters per minute of nitrogen, both beingmeasured as a gas. The intermediate beam was fed with 1.9 liters perminute of monosilane, 0.4 liters per minute of ethylene and 14.5 litersper minute of nitrogen, each being measured as a gas, and 0.0045 litersper minute of acetone, being measured as a liquid. The downstream beamwas fed with 0.8 liters per minute of monosilane, 0.2 liters per minuteof ethylene and 30 liters per minute of nitrogen, each being measured asa gas. The glass speed was 180 metres per hour and the glass thicknesswas 2 mm. The glass side reflection of the coating was measured as being70 to 72% using Illuminant D65 Source 1931 Observer conditions.

In Example 1 the gas flows were all measured at ambient temperature andpressure 0.7 bar, except for flows of nitrogen which were measured atambient temperature and 1 bar pressure, and acetone which is measured asa liquid, and all are quoted per metre width of glass coated.

No modification of the lehr conditions was required to anneal theresulting coated ribbon which had a highly reflecting appearance.

A mirror cut from the glass ribbon having dimensions of 160 mm×160 mmwas edge worked and cleaned and then supported, with the reflectingcoating being upwardly oriented, on a 2000×1000 mm piece of 6 mm floatglass. The glass assembly was then lowered onto a conveyor system of afluorine-doped tin oxide application plant having a coating apparatussimilar to that disclosed in GB 2225343. The glasses were conveyed intothe furnace of the plant and were held in the furnace for a time periodwhich was sufficient to raise the glass temperature to approximately600° C. The furnace was then fed from an upstream slot with 250 ml/minof tin tetrachloride in 25 m³ /hr of air as a carrier gas at atemperature of 250° C. At a downstream slot a mixture of HF andmethanol, and steam, all being in air as a carrier gas, were introducedinto the flow of tin tetrachloride. The HF/methanol mixture comprised4.76% by volume of methanol together with 95.24% by volume of a 20%solution of hydrofluoric acid. The steam was supplied at a rate of 11kg/hr and the carrier gas was applied at a rate of 120 m³ /hr with thetemperature being 450° C. The exhaust gases were extracted at a pressureof 0.3 inch of water gauge pressure.

A coating of fluorine-doped tin oxide around 3200 Angstroms thick wasdeposited on top of the reflecting coating. The sheet resistance of thecoated glass was measured at 14 ohms/square by using a 4 point probe.The reflection of the reflecting coating on the glass side was measuredat 70% using the same conditions specified above. Subsequently, silverbusbars 5 mm wide were silk screened along two opposed lengths of thecoated surface, the busbars being 137 mm apart. The printed assembly waspre-dried in an oven for 1 hour at 100° C. followed by firing atapproximately 500° C. in the coating furnace to consolidate the printedbusbars. Electrical connections were then made to the busbars by usinginsulated wires and indium as the solder.

The resistance across the busbars was then measured using a multimeteras 13.7 ohms. A voltage of 6 to 12 volts was applied across the busbarswith a current of from 0.47 to 0.9 amps. This established sufficientheating to produce a demisting surface.

The process and product of the preferred embodiments of the presentinvention have important advantages over the prior art. The processenables heatable glass mirrors to be produced "on line" in a singlemanufacturing process starting with the batch which is melted to producethe molten glass, which is formed into a continuous ribbon, coated withreflecting and heating layers, annealed and cut to size for subsequentstorage and for distribution. This is quite unlike the prior artprocesses used commercially for the production of heatable mirrors.which involve the initial production of glass panes cut from a ribbon,followed by a separate coating process (commonly carried out at adifferent location) on a separate production line, and then followed byassembly with a separate heating element which is in contact with anelectrically insulating film disposed between the heating element andthe silver reflecting layer of the mirror.

The present invention can provide the advantage that because thereflecting layer is insulating this obviates the requirement for anadditional insulating layer between the heating layer and the reflectinglayer as is required for the known silvered mirrors incorporatingheating assemblies.

What is claimed is:
 1. A method of producing heatable mirrors comprisingdepositing onto a ribbon of hot glass during the production process anon-metallic reflecting coating comprising a reflecting layer and atleast two reflection enhancing layers, the reflection enhancing layerscomprising an intermediate layer of the coating of relatively lowrefractive index and a layer adjacent to the intermediate layer ofrelatively high refractive index, the two layers other than theintermediate layer being outer and inner layers of the coating eachhaving a refractive index of at least 1.6, the intermediate layer havinga refractive index less than the refractive index of either said innerlayer or said outer layer and less than 3, at least one of said innerand outer layers being of silicon, the aggregate refractive index of theinner and outer layers being at least 5.5, whereby the mirrors so formedhave a visible light reflection in the range of 70% to 90%, anddepositing an electroconductive heating layer onto the mirrors.
 2. Amethod according to claim 1 wherein the heating layer is deposited overthe reflecting coating on the ribbon of hot glass during the productionprocess.
 3. A method according to claim 2 wherein the reflecting coatingis deposited onto the glass in the float bath of a float glass plant andthe heating layer is deposited onto the mirrors in a gap between thefloat bath and an annealing lehr.
 4. A method according to claim 2wherein the mirrors are back surface mirrors and the heating layercomprises the rear surface thereof.
 5. A method according to claim 1wherein the heating layer is deposited on a glass surface which is onthe opposing side of the glass to the reflecting coating.
 6. A methodaccording to claim 5 wherein the heating layer is deposited off-lineonto mirrors which have been cut from the coated ribbon.
 7. A methodaccording to claim 5 wherein the mirrors are front surface mirrors andthe heating layer comprises the rear surface thereof.
 8. A methodaccording to claim 1 wherein the reflecting layer comprises a layeradjacent to the intermediate layer of relatively high refractive index.9. A method according to claim 8 wherein at least one of the relativelyhigh refractive index layers is of silicon.
 10. A method according toclaim 9 wherein both of the relatively high refractive index layers areof silicon.
 11. A method according to claim 9 wherein the outer of thesaid layers of relatively high refractive index is of tin oxide,titanium oxide or a silicon oxide.
 12. A method according to claim 1wherein the layer of relatively low refractive index comprises a layerof silicon oxide.